[33.13] ISM-IGM Interaction and Edges of Disks

P. Maloney (CASA, Univ. of Colorado)

Galaxies do not exist in a vacuum. The outer, low-column
density edges of galactic disks can provide unique probes of
the environments in which they exist. The interactions of
these disks with their surroundings can be broadly divided
into two classes: photoionization and heating, and direct
physical interaction (ram pressure stripping and thermal
conduction).

The former process may involve either the extragalactic
ionizing background, or photons escaping from star-forming
regions in the galaxy itself. In either case there will be a
critical column density below which the gas is largely
ionized, rather than neutral. These two sources can be
distinguished by the variation of emission measure with
radius: this is predicted to be essentially independent of
radius for the case where the extragalactic background
dominates, while it should be a strong function of radius if
self-irradiation is important.

Direct physical interactions can also affect the physical
state of galactic disks. Galaxies embedded in a hot medium,
either the IGM or intragroup or intracluster gas, will be
subject to thermal conduction (modulo the influence of the
magnetic topology), which can lead to evaporation of cool
galactic gas, or to the formation of mixing layers. If the
relative velocity of the galaxy and the gas is substantial,
ram pressure stripping may be important, which will remove
gas below a critical surface density threshold.